Heavy rainfall may increase the infiltration rate, raising the groundwater table after a short period time. Variation in the hydrostatic pressure on potential sliding surfaces and trailing edges of fissures in a rock slope may contribute to slope failure. The failure mechanisms of rainfall-triggered rock wedge slides have been widely studied in the literature. However, studies on the dynamic evolution of groundwater in rock failure are still limited. In this study, a typical rock wedge slide that occurred in Chengkou County, Chongqing, China is selected as a case study to analyze the stability of rock wedge with double planes and tension crack subjected to groundwater dynamic evolution. Model tests were designed and performed to reproduce the dynamic evolution of groundwater in the rock slope and analyze its role in wedge failure. Development and distribution characteristics of water pressure are investigated through monitoring the water pressure distributed along the surfaces of wedge failures. From the experimental study, it is found that the initial formation process of water pressure along the sliding surface consists of four types of distribution patterns, i.e., triangular distribution, Type I trapezoidal distribution, Type II trapezoidal distribution, and Type III trapezoidal distribution. Based on the model tests and theoretical analysis, the order of the calculated safety factors of the Chengkou rock wedge slide are: triangle > type I trapezoid > type II trapezoid > type III trapezoid. The existence of inflection points in the trapezoidal distribution mode significantly increases water pressure on both sides of rock wedge, which contributes instability. The new approach for investigation of the effect of dynamic evolution groundwater on stability of rock wedge slide is developed based on fundamental mechanism and experimental study. It can be adopted for stability assessment of similar rock wedge slides in other areas.